Some vehicle engine systems utilize both direct in-cylinder fuel injection and port fuel injection. The fuel delivery system may include multiple fuel pumps for providing fuel pressure to the fuel injectors. As one example, a fuel delivery system may include a lower pressure fuel pump (or lift pump) and a higher pressure fuel pump arranged between the fuel tank and fuel injectors. The high pressure fuel pump may be coupled to the direct injection system, upstream of a fuel rail to raise a pressure of the fuel delivered to the engine cylinders through the direct injectors. However, when the high pressure fuel pump is turned off, such as when no direct injection of fuel is requested, pump durability may be affected, as the pump may be mechanically driven by the engine crank or camshaft. Specifically, the lubrication and cooling of the pump may be reduced while the high pressure pump is not operated, thereby leading to pump degradation.
To address this issue, various zero flow lubrication (ZFL) strategies may be applied. In one example approach, a zero flow lubrication strategy may use a known relation between the fuel rail pressure and the pump duty cycle for a given fluid at a given temperature condition. The learned transfer function is then used to determine a duty cycle to be output based on fuel rail pressure so as not to increase the fuel rail pressure when the determined duty cycle is applied. Specifically, the duty cycle applied may be adjusted to provide the desired lubrication to the high pressure pump without raising the rail pressure.
However the inventors herein have identified a potential issue with this approach. The transfer function is learned with the bulk modulus of the fuel system at the learning conditions to reduce part-to-part variability. For example, the transfer function may be learned with the bulk modulus at nominal conditions. However, when the learned transfer function is applied at conditions where the bulk modulus is different from the learning conditions, it can lead to errors. For example, the transfer function may be applied at non-nominal conditions. As such, the bulk modulus may change significantly with fuel temperature, fuel pressure, as well as fuel type. The errors may be such that the applied duty cycle does not provide the desired lubrication. Further, the error may raise the fuel pressure when it is desired not to. Overall, fuel pump performance is degraded.
In one example, the above issues may be addressed by a method for an engine fuel system comprising: learning a transfer function between duty cycle for a high pressure fuel pump and fuel rail pressure for a direct fuel injector for nominal bulk modulus conditions; and during conditions when not direct injecting fuel into an engine, operating the fuel pump with a duty cycle based on the learned transfer function and an instantaneous bulk modulus estimate. In this way, a transfer function for zero flow lubrication of a high pressure fuel pump may be learned more robustly, improving fuel pump lubrication.
As an example, in an engine system that is fueled via both port and direct injection, a high pressure pump may be used for increasing fuel pressure in a (direct injection) fuel rail connected to the direct injectors. A low pressure pump may be connected upstream of the high pressure pump and may provide pressure to port injectors on a different rail in addition to providing fuel to the high pressure pump inlet. During conditions when not direct injecting fuel into the engine, such as when only port injecting fuel into the engine, a zero duty cycle may be applied to the high pressure fuel pump for a duration to measure an average change in the direct injection fuel rail pressure over time due to fluctuations in fuel temperature. As such, this may be a reference (e.g., background) change in fuel rail pressure. Once the fuel rail pressure has normalized, and while still not direct injecting fuel into the spinning engine, a first duty cycle may be applied to the high pressure fuel pump and a resulting change in the direct injection fuel rail pressure may be recorded. The duty cycle of the high pressure pump may be incrementally changed in small amounts (e.g. 1%, 2%, 3%) and a fuel rail pressure data point may be recorded once the chamber pressure has stopped changing. A relationship between duty cycle and rail pressure can be learned at the present fuel conditions which may be different from nominal bulk modulus conditions. A duty cycle transfer function at nominal bulk modulus conditions may be adaptively learned based on the relationship by compensating for a variation in the fuel conditions at the time of the learning from the bulk modulus conditions. Then, during conditions when zero flow lubrication of the high pressure fuel pump is requested, such as when only port injecting fuel into the spinning engine, the learned transfer function at nominal bulk modulus conditions may be applied after adjusting the transfer function with a correction factor to compensate for differences between the nominal bulk modulus estimate and an instantaneous bulk modulus estimate. As such, the fuel bulk modulus at the time of zero flow lubrication may be different from the fuel bulk modulus at the time of learning the transfer function, both differing in varying degrees from the nominal fuel bulk modulus estimate. The instantaneous bulk modulus estimate may be determined as a function of fuel rail temperature, current fuel rail pressure and the type of fuel in the fuel rail. A duty cycle may be applied to the high pressure fuel pump based on the fuel-adjusted transfer function so as to not raise the fuel rail pressure and provide sufficient pump lubrication.
In this way, by learning a transfer function between duty cycle and rail pressure for a high pressure fuel pump, the robustness of zero flow lubrication controls for a high pressure fuel pump to adapt for various fuel systems and fuel types is improved. By determining the transfer function for nominal bulk modulus conditions and then applying the transfer function at non-nominal conditions while correcting for a difference between nominal bulk modulus and the bulk modulus at the time of applying the transfer function, duty cycle errors in ZFL control due to differences in fuel type and fuel system configuration are reduced. By enabling the transfer function to be learned at a first fuel condition and then be applied at a second, different fuel condition, zero flow lubrication can be easily adapted for various fuel systems and different fuel types without requiring the transfer function to be relearned at each fuel condition. Overall, the desired lubrication is achieved and the fuel pressure may not be raised above the desired pressure.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.